This invention relates, in general, to piston systems, such as continuously variable displacement pumps, engines, and compressors. Such devices are well known and many include a piston that reciprocates in a cylinder to achieve the pumping action. Many of these systems allow for varying the length of the piston stroke within the cylinder. These systems may include a movable member coupled to a drive shaft. The movable member is connected to the piston via a crankshaft, or similar member for varying the length of the piston stroke. In conventional devices, however, when the piston stroke is shortened, there often is a relatively large unswept volume in the cylinder. As used herein, an xe2x80x9cunswept volumexe2x80x9d is that section or volume inside the cylinder which is not reached by the piston at a given piston stroke. Large unswept volumes decreases the efficiency of the device. Therefore, what is needed is a device or method which controls or minimizes the unswept volume.